The parts came in the mail today! 

From left-to-right: 

A Fellowes 17-key number pad with PS/2 port. 

A no-name 500mAh 1S lithium-polymer battery from Amazon.

A Sparkfun Artemis Nano microcontroller. 

The microcontroller is really powerful for its size. According to the manufacturer, it can even run TensorFlow Lite models for voice recognition, etc. I chose it because it had several must-haves: native Bluetooth LE support, native Li-Po battery management, and was small enough to stuff into the case of the keypad. 

I think I set a new speedrun record in the Smoke% category. Less than half a minute after unboxing the controller, I plugged in the Li-Po and immediately smelled burning silicon.

My thought process went something like this: 

The Artemis Nano's got a JST socket. The battery's got a JST plug. Should work.

Turns out all the fancy one-way keying on the socket doesn't do jack-shit if the plug is crimped backwards.

Lesson number one: NEVER TRUST THE CONNECTOR.

I unplugged it fast enough that the charge controller most likely survived. I'll rewire the Li-Po tomorrow, and hope nothing else blows up. 

- TK